1. Field of the Invention
The present invention relates generally to semiconductor electromagnetic radiation detectors. In particular, the present invention relates to an improved detector incorporating a higher bandgap layer under the point of electrical contact to retard minority carriers and enhance the sensitivity of the detector.
2. Description of the Prior Art
Photosensitive semiconductors are basically of two types, photoconductive and photovoltaic. When radiation of the proper energy falls upon a photoconductive semiconductor, the conductivity of the semiconductor increases. Energy supplied to the semiconductor causes covalent bonds to be broken, and electron-hole pairs in excess of those generated thermally are created. These increased current carriers decrease the resistance of the material. This "photoconductive effect" in semiconductor materials is used in photoconductive detectors.
If, on the other hand, the semiconductor sensor is such that it incorporates a pn junction, it gives rise to electron-hole pairs which create a potential difference in response to radiation of the proper energy. This is referred to as a "photovoltaic" effect. The semiconductor electromagnetic radiation detectors of the present invention are photoconductive detectors.
A photoconductive detector can be a bar of semiconductor material having electrical contacts at the ends. In its simplest use, the photoconductive detector is connected in series with a direct-current voltage source and a load resistor. The change in resistivity of the photoconductive detector in response to incident radiation is sensed.
The present invention deals primarily with increasing the photosensitivity or responsivity of the detector to the electromagnetic wavelengths corresponding to the application of the device. It has been discovered that the best method to accomplish this is to reduce the rate of annihilation of the minority carriers to thereby increase or maximize the lifetime of the minority carriers generated by absorbed incident photons so that the excess majority carriers continue to flow for an increased length of time.
In general, most of the minority carriers are annihilated in one of three basic areas. These are within the detector bulk, at the surfaces of the detector including the surface exposed to the incident radiation, and at the electrical contacts to the detector.
The prior art teaches methods to reduce the rate of annihilation of the minority carriers within the detector bulk. One well known method for detectors of visible radiation is to incorporate impurities or lattice defects into the detector material which act to temporarily trap the minority carriers and thus reduce the rate of annihilation. Such a technique is described in R. B. Bube, "Photoconductivity of Solids", New York (1960), at p. 69.
The prior art also teaches methods to prevent the minority carriers from recombining at the surface areas such as at the back of the detector or at the area exposed to the incident radiation by chemically or physically treating the surfaces or depositing a thin layer of a chemical compound on the surface. This is known as "passivation". Typically, the treatment creates electronic states on the surface which create an energy barrier that repels the minority carriers.
Unfortunately the prior art methods of passivating the surface create a layer at the surface that prevents or interferes with the formation of a good electrical contact. The existence of a good electrical contact means that majority carriers can be efficiently injected from the contact into the detector with no barrier or excessive resistance to their flow. Accordingly, either the surface has not been treated in the area in which the electrical contacts are placed, or the surface layer that is formed has been removed from the contact area prior to the formation of the electrical contacts. Thus, treatment has not been made directly under the electrical contacts.
An alternate approach to the protection of the surface exposed to the incident radiation in photovoltaic devices is to incorporate a layer of higher bandgap material at the sensitive area exposed to the incident radiation. Such a technique is described in the U.S. Pat. No. 4,132,999 issued Jan. 2, 1979. By that technique, however, the electrical contacts are caused to penetrate through the higher bandgap layer to the infrared sensitive material so that again there is no protection under the contacts. Yet another prior art approach involves the incorporation of a layer of higher doped material at the surfaces. That approach is described in U.S. Pat. No. 4,137,544 issued Jan. 30, 1979. As with the last described technique, however, the higher doped layer does not extend under the active portion of the electrical contacts.
A different prior art approach is described by M. A. Kinch et al (Infrared Physics, Volume 17, pp. 137-145, 1977) in which the incident radiation is absorbed in a region that is not contiguous with the electrical contacts, i.e., geometrical separated from the electrical contacts. In this manner, the minority carriers that are generated by the incident radiation take a longer time to reach the contacts, thereby increasing the average life of the minority carriers.
In the case of infrared photoconductive detectors, operation is frequently in what is called the "sweepout" mode wherein the electrical contacts are placed quite close together on the semiconductor material. In this mode, the electric field applied to the detector moves the minority carriers to the electrical contacts in a very short length of time. The time that the minority carriers take to reach the electrical contact is then so short that the possibility of the minority carriers being annihilated within the detector material is greatly reduced. Additionally, the effect of annihilation of minority carriers at the surfaces of the detector is also reduced. This rapid rate of annihilation of minority carriers by recombination at the electrical contact area can limit the responsivity of infrared detectors in general, and typically will limit the responsivity severely when the photoconductive detector is operated in the sweepout mode.
The full significance of this phenomenon has not generally been appreciated in the prior art. According to the present invention, it has been discovered that reducing the annihilation of minority carriers directly under the contact can increase the responsivity of the detector far more than was previously recognized.
A prior art attempt at a related solution is described by Y. J. Shacham-Diamand and I. Kidron (Infrared Physics, Vol. 21, p. 105, 1981) as a method to improve the responsivity by reducing the rate of recombination of minority carriers in the immediate vicinity of the electrical contacts to the semiconductor. This was accomplished by incorporating a built-in electric field opposing the collection of minority carriers under the cathode contacts. The built-in electric field resulted from the diffusion of donors into the Hg.sub.1-x Cd.sub.x Te semiconductor which, in turn, resulted in a gradient in the electron concentration over about a two-micron region beneath the contact. Thus, the region under the contact was made n.sup.+ if the detector material was n-type. Although some increase in responsivity of the detectors has been achieved by this approach, it has some inherent drawbacks. A disadvantage of this approach is that the added n.sup.+ dopant leads to a decreasing minority carrier lifetime in the contact region which makes the system self-limiting in responsivity. In addition, the built-in electric field is limited by the solubility limit of the n.sup.+ dopant which reduces the blocking effect on the minority carriers.
Thus, the prior art has provided means for improving the responsivity of infrared detectors by reducing the rate of annihilation of minority carriers within the bulk of the infrared detector materials, and by preventing annihilation of minority carriers at surfaces of the detector that are not used for making electrical contacts to the detector. The prior art has further disclosed means to prolonging the lifetime of the minority carrier by using a geometry in which the electrical contacts are placed at an increased distance from the absorbed incident radiation. However, the prior art has not disclosed a truly effective means of improving infrared detector response by reducing the rate of annihilation of minority carriers upon reaching the electrical contact.